The invention generally relates to an all wheel drive for a motor vehicle having a plate clutch, and more particularly, to a plate clutch having different rotational moment transfer characteristic as a function of the rotational speed differences of the axles.
All wheel drive vehicles having an intermediate differential positioned between the front and rear axles have the advantage of avoiding bracing or stressing in the drive train when the vehicle is negotiating a curve or when the individual tires are experiencing different driving conditions. The resulting slippage is accommodated by the differential, thereby improving the driving conditions of the vehicle.
The intermediate differentials, which are typically constructed as planetary gear differentials, should be configured such that during acceleration, the axle load shifting will transfer more driving force to the rear axle than to the front axle. Because the vehicle acceleration along with the motor load and the friction between the tires and the driving surface are controlling and the momentary load conditions of the vehicle also affect the axle load distribution, there is only one fully corresponding acceptable distribution relationship between the driving force at the front and rear axles which is optimal for each operating condition. As a compromise, the distribution relationship of the intermediate differential is designed for an intermediate frictional value for the tires and an intermediate loading condition, with the effect on the driving condition and the load change relationship when driving through curves also being taken into account.
In the all wheel drive described in German Published Unexamined Application (DE-OS) No. 32 12 495 an intermediate differential is provided as a planetary gear transmission which distributes the driving moment between the front axle with respect to the rear axle according to the relationship of 1.25 to 1. Because such an intermediate differential can deliver only a single driving force distribution this arrangement fails to accommodate for a plurality of operating conditions.
In another German Published Unexamined Application (DE-OS) No. 22 09 879, a system attempting to correct these drawbacks is described, in which a fluid friction clutch is arranged in the power flow or power path parallel to the intermediate differential. Such a clutch builds the resulting rotational moment as a function of the rotational speed difference of the clutch parts, for example, with a viscous lamella or plate clutch from the outer plates fixed to a housing with respect to hub fastened inner plates. The rotational moment with respect to the rotational speed difference has a representative characteristic curve which shows a strong degressive characteristic. The rotational moment increases in the region of very small velocity differences at a steep slope. This degressive characteristic, however, is a great disadvantage when negotiating sharp curves with the vehicle. Even if the rotational moment is changed with respect to the rotational speed difference such that the representative characteristic curve is flatter, the viscous lamella or plate clutch could then no longer fully transfer the motor rotational moment to the axle driving on a dry surface from the slipping axle driving on an icy surface, thereby resulting in the viscous lamella or plate clutch becoming overheated and either damaged or destroyed.
Depending on the resultant unequal distribution of the driving force at the rear axle and front axle with the intermediate differential, the slip moments differ depending upon whether the front axle or the rear axle is on a firm road surface.
It is, therefore, an object of the present invention to provide a lamella or plate clutch that can facilitate the driving if one axle is on a slippery road surface and the other axle is on a solid grippable road surface.
The above and other objects are attained by a plate clutch which provides different characteristics depending upon whether the front axle or the rear axle is slipping.
In specially preferred embodiments of the invention, the plate clutch includes a plurality of inner plates and outer plates which alternatively engage between one another. A portion of the plurality of inner plates are rotationally fixed to a driven clutch hub with the remaining plurality of the inner plates being connected to the clutch hub by means of a one way clutch. As a result of this arrangement, the latter portion of inner plates are only engaged in one rotational direction of the clutch hub for transfer of rotational moment.
Certain preferred embodiments of the invention use a plate clutch having mechanical friction connections, other preferred embodiments of the invention utilize a viscous plate clutch which is a plate clutch having rotational moment transfer being effected via fluid friction.